1. Field of the Invention
The present invention is directed to a process for the preparation of built-up non-cast silicone rollers which do not exhibit build lines.
2. Description of the Related Art
Polymer faced rollers have long been used in industry. The polymer facings reduce noise, confer chemical resistance, and cushion the contact of the roller against surfaces which it contacts. Many polymers have been used for such “faced rollers,” including in particular, polyurethanes.
Silicones are a special class of polymers which have been used to make faced rollers. Silicones are available in a wide range of hardnesses, ranging from very soft elastomers to rigid materials. Moreover, silicones have physical and chemical characteristics which make them particularly well suited for certain roller applications. For example, silicones are very chemically resistant, and are highly resistant to thermal stress as well, providing a higher operating temperature than many commonly used polymer facings, such as those of polyurethanes. Moreover, silicones tend to have high releasability, and have been used in thin layer form (coatings) for many years as release coatings for pressure sensitive adhesives. Thus, silicones have found a valuable place in the faced-roller market.
While small diameter rollers may be faced with silicones by conventional thermoplastic or thermosetting techniques such as casting and injection molding, large diameter rollers or long rollers and like products cannot be made by such processes. Rather, such rollers are prepared in composite structures, for example by spiral wrapping of curable silicone rubber around the periphery of the roller, or by wrapping the entire periphery of the roller with succeeding layers of thin, curable silicone rubber. These techniques are shown in FIGS. 1 and 2, where FIG. 1 illustrates spiral wrapping of an extruded strip of silicone rubber 2 around metal core or “boss” 1, and wherein FIG. 2 illustrates concentric wrapping. Other wrapping methods are also possible.
The roller boss (1) is generally of metal, although it may be made of other materials as well, for example thermoplastic or thermosetting polymers, wood, ceramic, etc. For reasons of cost, durability, and physical properties such as modulus and freedom from distortion, metals such as aluminum or steel are commonly used.
The boss or other substrate is generally treated to facilitate adherence of the silicone to the boss. For example, the surfaces of the metal boss may be knurled or otherwise machined, may be grooved, sometimes with grooves which are narrower at the surface of the roller than further toward the axis or center of the roller (“undercuts”), or may be treated by blasting, e.g. sand blasting. Since the boss surfaces can be obscured or substantially obscured after application of an opaque or semi-opaque silicone, care in preparing an aesthetic surface on the boss can be dispensed with. This is the customary procedure; filled and pigmented silicone rubbers are used to face the roller, thus obscuring the rough surface. The pigments may also be used to provide an aesthetic or distinguishing color. Avoiding fine machining or finishing of the boss allows for economy of manufacture without loss of adhesion, for which the surface treatment is used. Following facing of the boss with the curable silicone rubber, the composite is heated or otherwise treated to cure the silicone rubber. This may be achieved by numerous methods well known in the art. For example, the surface may be placed under modest pressure by a suitable mold or pressure bladder and cured at elevated temperature. Following cure, the now integral silicone facing is machined to present a surface which is rotationally uniform and of the desired smoothness. Frequently, the uncured, built-up roller is tightly wrapped with a polymer fabric, and then heated or “autoclaved.” The increase in diameter due to thermal expansion is sufficient to place the silicone layer(s) under pressure against the fabric, facilitating consolidation. The silicone rubber merges or coalesces where adjacent strips or layers abut, creating an integral silicone facing.
The silicones used in such applications are generally pigmented and/or filled. The pigmentation may be used to produce a recognizable color and to obscure the boss surface, while fillers act both to lower the cost of the facing by replacement of relatively expensive silicone with inexpensive filler, and also to increase wearability of the facing. It has been found, however, that silicone roller facings containing pigments and fillers exhibit so-called “build lines” at what were the edges of the silicone rubber prior to curing/consolidation. These build lines are often observable visually as a difference in height or color near what were the edges of the uncured silicone facing. Even after machining, latent build lines exist which manifest themselves particularly when the rollers are used in an environment where solvents are employed. The silicone tends to swell under such conditions, but the swelling is not uniform across the surface. Rather, the swelling at the build line positions 3 (FIGS. 1, 2) is different from swelling at the remaining portions 4 of the roller. Thus these portions of the roller will exert different pressure on substrates moved under the roller at the build line positions. This difference in pressure may be then manifested in a product produced by use of such rollers as elevations or depressions in the product surface.
It would be desirable to produce a silicone faced roller by wrapping techniques which do not exhibit build lines.